Recently, in the technical field of alternating-current-motor such as industrial devices, household appliances, and cars, there are increasing cases of using a system that drives a permanent-magnet synchronous motor by an inverter, in place of a conventional system that drives an induction motor by an inverter.
The permanent-magnet synchronous motor is known as a highly efficient motor as compared with the induction motor for the following reasons: because a magnetic flux of a permanent magnet is established, an excitation current is not necessary; because no current flows to a rotor, a secondary copper loss does not occur; and because a reluctance torque using a difference of magnetic resistance of a rotor is used in addition to a torque generated by a magnetic flux of a permanent magnet, a torque can be effectively obtained. Thus, application of the permanent-magnet synchronous motor to a power conversion device to drive an electric vehicle has been also studied in recent years.
Examples of a method of drive-controlling a permanent-magnet synchronous motor include a maximum-torque/current control for generating a maximum torque at a certain current and a maximum efficiency control for maintaining maximum efficiency of the motor. These optimum control methods are a method of adjusting current amplitude and a phase to be applied to the motor to become optimum values, which are stored in arithmetic expressions and tables in advance. Because details of these methods are disclosed in various documents, detailed explanations thereof will be omitted here. The maximum-torque/current control is disclosed in Patent Document 1 mentioned below, for example.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-33097
In performing the optimum control methods mentioned above, a torque current (a q-axis current) and a magnetic-flux current (a d-axis current) are adjusted to optimum values corresponding to rotation speed of the motor and a magnitude of an output torque. Therefore, an optimum interlinkage flux of the motor changes corresponding to the rotation speed of the motor and the magnitude of the output torque, and a voltage between the motor and a terminal (=inverter output voltage) varies greatly.
Further, a voltage of a direct-current power source that becomes an input to an inverter incorporated in a power conversion device used to drive an electric vehicle is about 1500 volts to 3000 volts, which is a higher voltage than that used for general industrial applications. A high-withstand-voltage switching element having a withstand voltage of about 3300 volts to 6500 volts is used for the inverter. However, the high-withstand-voltage switching element has a large switching loss and a large conduction loss. An inverter loss that is a sum of these losses becomes in the order of several kilowatts to 10-odd kilowatts. Accordingly, the magnitude, weight, and cost of a cooler constituted by a radiator and a cooling fan to cool down the loss occupy a significant part of the power conversion device.
Therefore, preferably, a switching frequency is designed to be as low as possible within a range not generating current oscillation, torque pulsation, noise, and vibration of the motor, and to minimize the inverter loss to provide a small cooler. Specifically, a normal switching frequency is set to around 750 hertz, and the cooler is preferably configured to have a capacity capable of cooling down the inverter loss by the switching frequency. Because the radiator and the switching element have a thermal capacity, the switching frequency can be increased to around 1000 hertz for a short period of time.
Meanwhile, regarding a polar number of a permanent-magnet synchronous motor of which an inverter is to be controlled, six or eight poles are suitable to drive the electric vehicle from the viewpoint of reducing the size and weight of the motor. This polar number is larger than four, which is the case of the majority of conventional induction motors. When a motor has eight poles, a maximum value of an inverter output frequency (an inverter output frequency at designed maximum speed of the electric vehicle) becomes about 400 hertz, and this is about twice of that when a conventional induction motor is used.
For example, when a motor is operated by setting an inverter output frequency to 400 hertz in a state that a switching frequency is 750 hertz, a pulse number included in a half cycle of an inverter output voltage becomes 1.875, which is very small and obtained by dividing a carrier frequency (=switching frequency) by the inverter output frequency. When the motor is driven in this state, a pulse number and a pulse position included in a positive half cycle and a negative half cycle, respectively of the inverter output voltage, become unbalanced. Consequently, positive-and-negative symmetry of the voltage applied to the motor is collapsed, and current oscillation and torque pulsation are generated in the motor, and they become a cause of noise and vibration.
The following arrangement is one idea to avoid this phenomenon. That is, in a region of a high inverter-output frequency as a region where a pulse number decreases, a carrier frequency is determined by synchronizing it with the inverter output frequency, thereby securing positive-and-negative symmetry of the voltage applied to the motor by setting the same the pulse number and the pulse position, respectively included in each of the positive half cycle and the negative half cycle of the inverter output voltage.
For example, as a setting capable of adjusting an output voltage amplitude of an inverter and also setting a switching frequency as low as possible, a so-called synchronous three-pulse mode having a carrier frequency selected to three times of the inverter frequency can be considered. In this case, under a condition of an inverter output frequency being 400 hertz, a carrier frequency (switching frequency) becomes 1200 hertz.